net: New iface_stat module to monitor persistent network stats

Adding a new module that tracks the number of bytes/packets transfered
by a network interface, even after the interface has been
removed. This is relevant to track the total data usage in mobile
devices whose interfaces are added and removed quite frequently (WiFi,
Bluetooth, 3G,..).

Monitoring is done only for devices that are configured with a valid
IP address (check to exclude virtual/loopback/tunnel interfaces).

Change-Id: I8ac642af1990433ebd0784e8dbd72bf0714b5bf6
Signed-off-by: Ashish Sharma <ashishsharma@google.com>